The constantly increasing demand for high data rates in cellular networks requires new approaches to meet this expectation. A challenging question for operators is how to evolve their existing cellular networks so as to meet the requirement for higher data rates. In this respect, a number of approaches are possible: i) increase the density of their existing macro base stations, ii) increase the cooperation between macro base stations, or iii) deploy smaller base stations in areas where high data rates are needed within a macro base stations grid. This last option is referred to in the related literature as a “Heterogeneous Network”, or “Heterogeneous Deployment” and the layer consisting of smaller base stations is termed a “micro”, or “pico” layer.
Building a denser macro base station grid, while simultaneously enhancing the cooperation between macro base stations (hence either using options i) or ii) above) is definitely a solution that meets the requirement for higher data rates; however such an approach is not necessarily a cost-efficient option, due to the costs and delays associated with the installation of macro base stations especially in urban areas where these costs are significant.
In this landscape, the solution of deploying small base stations within the already existing macro layer grid is an appealing option, since these smaller base stations are anticipated to be more cost-efficient than macro base stations, and their deployment time will be shorter as well. However, such a dense deployment of macro base stations would lead to a significantly higher amount of signaling due to frequent handovers for users moving at high speeds.
In contrast, the macro layer grid of a heterogeneous network can serve users moving at high speed, as well as service wider areas where the demand for high data rates is less and the grid consisting of smaller base stations in the heterogeneous network can be employed to service areas having a higher density of users requiring high data rates, or hotspots as these areas are termed.
One of the main targets of low power nodes is to absorb as many users as possible from the macro layers. In theory, this helps to offload the macro layer and allow for higher data rates in both the macro and in the pico layer.
In this respect, several techniques have been discussed and proposed within 3GPP:
extending the range of small cells by using cell specific cell selection offsets
by increasing the transmission power of low power nodes; and
by simultaneously setting appropriately the UL power control target PO for the users connected to low power nodes.
The problem with heterogeneous networks is that small base stations, even if they are easier to deploy and operate than macro base stations, still cannot be deployed everywhere since there are restrictions on where to place them. Furthermore, often the placement of small base stations or relay nodes, results in insufficient coverage for all of the users targeted to be served. Hence, even after the addition of small base stations around them, there still exists the possibility of users being in coverage holes of the macro layer, and as such they do not necessarily benefit from this addition of small base stations, relays, or low power nodes, in general. This can happen due to an obstacle, such as a wall, or similar barrier being between the low power node and the user in the macro layer coverage hole.
Moreover, a situation like the one described above might occur even in the case of significant obstacles between the low power nodes and certain users. Due to the much higher power transmitted by the macro base stations, low power nodes do not always succeed in absorbing many users. For example, there might be cases such as the macro layer not providing good coverage to a certain area, and thus users in this area could still connect to the macro base station rather than to the low power node nearby.
There are also other occasions where the addition of low power nodes does not yield the desired result. Consider the case where during certain time periods of either a day, or a week, there is a high concentration of users within a given geographical area. There could be a number of reasons that would cause an operator to not deploy a sufficient amount of macro base stations in such a coverage area. Reasons include the possibilities that base station sites are very costly to obtain in this area or the morphology of the area is such that it does not facilitate the provision of high data rates to users therein. Another reason might be that adding macro base stations is not profitable in this area, considering that this area might be underutilized for a significant percentage of the day, or during the week. The operator could then decide to deploy low power nodes within this area. Due to the difficulties in finding sites, or due to the cost analysis performed, it could be decided to deploy a limited amount of low power nodes in this area. Furthermore it may be decided to extend the range of the low power nodes in order to absorb as many users as possible in the area. By doing so, two primary effects will be observed in the downlink i) users connected to lower power nodes due to this range expansion could experience worse link conditions than on the link to the macro base station and ii) a higher number of users share the pool of resources within the small cell.
Cell selection is done typically on the basis of reference symbols received power (RSRP) in the downlink (DL) reference symbols. Typically, a handover (HO) from a serving cell to a neighbouring cell occurs when the RSRP from the neighbouring cell, RSRPneighbor is higher than the RSRP from the serving cell, RSRPserving, serving plus an HO margin (used so as to avoid ping-pong HOs), plus a cell selection offset:RSRPneighbor≧RSRPserving+HO margin+Offset
When trying to expand the range of low power nodes (small cells), so as to facilitate HOs from the serving macro cells to neighbor pico cells, the value of the Offset above is set to negative values. This means that the UE is connected to a low power node (LPN), even if the RSRP from this LPN is lower than the RSRP from the macro cell. This has a consequence that the UE receives lower quality signal in the DL, than the UE would have received if the UE had remained at the macro cell coverage.
Note that in the uplink, users at the extended range of small cells experience a better link than the one experienced within the macro cell but effect ii) still applies in this direction as well. The combination of these two effects mentioned above possibly results in the average data rate per user in the downlink not being substantially higher than that of the macro cell or probably, the average data rate per user is not high relative to the desired level.